1,3-Bis(3,5-dichlorophenyl) urea compound 'COH-SR4' inhibits proliferation and activates apoptosis in melanoma

Article abstract of DOI:10.1016/j.bcp.2012.08.020

The current clinical interventions in malignant melanomas are met with poor response to therapy due to dynamic regulation of multiple melanoma signaling pathways consequent to administration of single target agents. In this context of limited response to single target agents, novel candidate molecules capable of effectively inducing tumor inhibition along with targeting multiple critical nodes of melanoma signaling assume translational significance. In this regard, we investigated the anti-cancer effects of a novel dichlorophenyl urea compound called COH-SR4 in melanoma. The SR4 treatment decreased the survival and inhibited the clonogenic potential of melanomas along with inducing apoptosis in vitro cultures. SR4 treatments lead to inhibition of GST activity along with causing G2/M phase cell cycle arrest. Oral administration of 4 mg/kg SR4 leads to effective inhibition of tumor burdens in both syngeneic and nude mouse models of melanoma. The SR4 treatment was well tolerated and no overt toxicity was observed. The histopathological examination of resected tumor sections revealed decreased blood vessels, decrease in the levels of angiogenesis marker, CD31, and proliferation marker, Ki67, along with an increase in pAMPK levels. Western blot analyses of resected tumor lysates revealed increased PARP cleavage, Bim, pAMPK along with decreased pAkt, vimentin, fibronectin, CDK4 and cyclin B1. Thus, SR4 represents a novel candidate for the further development of mono and combinatorial therapies to effectively target aggressive and therapeutically refractory melanomas.

Full text of DOI:10.1016/j.bcp.2012.08.020

Biochemical Pharmacology 84 (2012) 1419–1427
Contents lists available at SciVerse ScienceDirect
Biochemical Pharmacology
journal homepage: www.elsevier.com/locate/biochempharm
1,3-Bis(3,5-dichlorophenyl) urea compound ‘COH-SR4’ inhibits proliferation and
activates apoptosis in melanoma
a
a
a
a
Sharad S. Singhal a, , James Figarola , Jyotsana Singhal , Kathryn Leake , Lokesh Nagaprashantha ,
*
Christopher Lincoln b, B. Gabriel Gugiu c, David Horne b, Richard Jove b, Sanjay Awasthi a, Samuel Rahbar a
a Department of Diabetes and Metabolic Diseases Research, Beckman Research Institute, City of Hope, Comprehensive Cancer Center, Duarte, CA 91010, United States
b Department of Molecular Medicine, Beckman Research Institute, City of Hope, Comprehensive Cancer Center, Duarte, CA 91010, United States
c Department of Mass Spectrometry & Proteomics Core, Beckman Research Institute, City of Hope, Comprehensive Cancer Center, Duarte, CA 91010, United States
A R T I C L E I N F O
A B S T R A C T
Article history:
The current clinical interventions in malignant melanomas are met with poor response to therapy due to
dynamic regulation of multiple melanoma signaling pathways consequent to administration of single
target agents. In this context of limited response to single target agents, novel candidate molecules
capable of effectively inducing tumor inhibition along with targeting multiple critical nodes of
melanoma signaling assume translational significance. In this regard, we investigated the anti-cancer
effects of a novel dichlorophenyl urea compound called COH-SR4 in melanoma. The SR4 treatment
decreased the survival and inhibited the clonogenic potential of melanomas along with inducing
apoptosis in vitro cultures. SR4 treatments lead to inhibition of GST activity along with causing G2/M
phase cell cycle arrest. Oral administration of 4 mg/kg SR4 leads to effective inhibition of tumor burdens
in both syngeneic and nude mouse models of melanoma. The SR4 treatment was well tolerated and no
overt toxicity was observed. The histopathological examination of resected tumor sections revealed
decreased blood vessels, decrease in the levels of angiogenesis marker, CD31, and proliferation marker,
Ki67, along with an increase in pAMPK levels. Western blot analyses of resected tumor lysates revealed
increased PARP cleavage, Bim, pAMPK along with decreased pAkt, vimentin, fibronectin, CDK4 and cyclin
B1. Thus, SR4 represents a novel candidate for the further development of mono and combinatorial
therapies to effectively target aggressive and therapeutically refractory melanomas.
Received 18 July 2012
Accepted 21 August 2012
Available online 28 August 2012
Keywords:
Melanoma
SR4
Syngeneic model
Tumor xenografts
ß 2012 Elsevier Inc. All rights reserved.
1. Introduction
growth factor phosphorylation to enhance mitogenic signaling in
transforming melanoma cells [5].
Melanoma arises from the malignant transformation of the
pigmented cells of the skin called melanocytes. The incidence of
cutaneous melanoma has been rising in the past decade.
Melanoma represents only 4% of all skin cancers, but causes
nearly 80% of skin cancer related deaths [1]. Melanoma has a
positively skewed risk towards fair skinned and young individuals
[2]. Metastatic melanoma is an almost certain death sentence
because of the highly refractory nature of this malignancy [3]. The
transformation of normal melanocytes in to melanoma cells is
initiated by a complex spectrum of interacting genetic and
environmental factors, some of them having the potential to
trigger oncogenic cascade alone. The established risk factors for
melanoma include exposure to UVB rays [4]. The UVB activates
The cellular inputs that reprogram melanoma signaling are
modulated by a complex set of cytoplasmic and nuclear networks.
Metastatic melanoma remains a daunting therapeutic challenge
because of simultaneous dysregulation of multiple important
signaling pathways that suppress apoptosis and promote growth
and invasion [6]. A majority of the patients with metastatic
melanoma with refractoriness to therapy have V600E mutations in
BRAF which leads to continuous activation of the MAPK pathway
[7,8]. BRAF targeted therapy has recently emerged as the most
effective therapy for melanoma, but response rates are less than
desirable and survival advantage is relatively short [9]. Melanoma is
a cell type typically very rich in mercapturic acid pathway enzymes
like glutathione S transferases (GSTs) which play a vital role in the
survival, apoptosis and drug-sensitivity of melanomas [10].
Currently there are no effective chemotherapeutic strategies
that are safe, affordable and effective given the limited treatment
options available for advanced stages of melanoma [11]. Hence, the
development of novel targeted therapy assumes immense
Abbreviation: COH-SR4, 1,3-bis(3,5-dichlorophenyl)urea ‘‘City of Hope compound’’.
* Corresponding author. Tel.: +1 626 256 4673x31238; fax: +1 626 301 8136.
E-mail address: ssinghal@coh.org (S.S. Singhal).
0006-2952/$ – see front matter ß 2012 Elsevier Inc. All rights reserved.
http://dx.doi.org/10.1016/j.bcp.2012.08.020

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S.S. Singhal et al. / Biochemical Pharmacology 84 (2012) 1419–1427
translational significance in contemporary melanoma research.
Based on our screening to identify novel products with anti-cancer
activities we have identified the anti-cancer effects of a dichlor-
ophenyl urea compound called ‘‘COH-SR4’’ in various cancers. SR4
is 1,3-bis(3,5-dichlorophenyl)urea, which has shown promising
anti-cancer activity against human leukemia cells [12]. In this
study, following initial in vitro studies in melanoma cultures, we
studied the anti-cancer effects and the respective mechanisms of
action of SR4 in syngeneic and nude mouse models of melanoma.
and complete medium (i.e. withFBS)was added. After24 hsilencing,
cells were exposed with SR4. After 96 h incubation, MTT assays as
well as Western blot analyses for GSTp expression were performed.
2.5. Cell survival (MTT) assay
Cell density measurements were performed using a hemocy-
tometer to count reproductive cells resistant to staining with trypan
blue. Approximately 20,000 cells were plated into each well of 96-
well flat-bottomed micro-titer plates. After 12 h incubation at 37 8C,
2. Materials and methods
medium containing SR4 (ranging 0–200
cells. After 96 h incubation, 20 L of 5 mg/mL MTT were introduced
to each well and incubated for 2 h. The plates were centrifuged and
medium was decanted. Cells were subsequently dissolved in 100
mM) were added to the
m
2.1. Reagents
mL
Terminal deoxynucleotidyl-transferase deoxyuridine triphos-
phate nick-end labeling (TUNEL) fluorescence and avidin/biotin
complex (ABC) detection kits were purchased from Promega
(Madison, WI) and Vector (Burlingame, CA), respectively. MTT,
Horseradish peroxidase (HRP)-conjugated anti-mouse, and anti-
rabbit secondary antibodies were procured from Sigma (St. Louis,
dimethyl-sulfoxide withgentleshakingfor 2 h atroomtemperature,
followed by measurement of optical density at 570 nm [13].
2.6. TUNEL apoptosis assay
For TUNEL assay, 1 Â 105 cells were grown on the cover slips for
MO). PARP,
b
-actin, fibronectin, vimentin, Bim, Bcl2, cyclin B1,
ꢀ12 h followed by treatment with SR4 (10
mM) for 24 h. Apoptosis
CDK4, Akt, pAkt (S473), GAPDH, Ki67, CD31 and pAMPK (T172
)
was determined by the labeling of DNA fragments with terminal
deoxynucleotidyl-transferase dUTP nick-end labeling (TUNEL)
assay using Promega fluorescence apoptosis detection system
according to the protocol described previously [14,15].
antibodies were purchased from Santa Cruz Biotechnology
(Columbus, OH) and Cell Signaling Technologies (Danvers, MA).
GST
p siRNA (GSTP1_1 FlexiTube siRNA; SI00300349) was pur-
chased from Qiagen (Valencia, CA).
2.7. Flow cytometry analysis of cell cycle regulation
2.2. Synthesis of SR4
2 Â 105 cells were treated with SR4 (10
mM) for 18 h at 37 8C.
The
1,3-bis(3,5-dichlorophenyl)urea
compound
SR4
After treatment, floating and adherent cells were collected, washed
with PBS, and fixed with 70% ethanol. On the day of flow analysis,
cell suspensions were centrifuged; counted and same numbers of
(C13H8Cl4N2O, MW = 350.03) was synthesized according to a
previously validated protocol by Dr. Christopher Lincoln, Director
of Chemical GMP Synthesis Facility, Beckman Research Institute,
City of Hope National Medical Center, Duarte, CA [12]. Briefly, 3,5-
dichlorophenyl isocyanate (1.21 g (96%), 6.17 mmol) was added
portion-wise to a stirring solution of 3,5-dichloroaniline (1.00 g
(98%), 6.33 mol) in dichloromethane (15 mL) under N2. After 19 h at
ambient temperature, the entire reaction mixture was filtered and
the filter cake was washed with dichloromethane (2Â 10 mL). The
solid was dried in vacuo to afford 1,3-bis(3,5-dichlorophenyl)urea
(1.78 g, 82%) as a white crystalline solid. 1H NMR (400 MHz, DMSO-
cells were resuspended in 500
Cells were then incubated with 2.5
at 37 8C for 30 min after which they were treated with 10
m
L PBS in flow cytometry tubes.
L of RNase (stock 20 mg/mL)
L of
m
m
propidium iodide (stock 1 mg/mL) solution and then incubated at
room temperature for 30 min in the dark. The stained cells were
analyzed using the Beckman Coulter Cytomics FC500, Flow
Cytometry Analyzer. Results were processed using CXP2.2 analysis
software from Beckman Coulter.
d6)
d
9.35 (s, 2H), 7.53 (d, J = 1.8 Hz, 4H), 7.17 (t, J = 1.8 Hz, 3H); 13
152.0, 141.8, 134.1, 121.3, 116.7;
C
2.8. Analysis of SR4 in serum by mass spectrometry
NMR (100 MHz, DMSO-d6)
d
HRMS-ESI (m/z (%)) 348.9278 (100), 346.9310 (73), 350.9255 (48),
Mice (n = 3 each for control and SR4 treatment) were
349.9323 (10), 352.9239 (8), 347.9357 (7), 351.9303 (4).
administered either 0.2 mL corn oil or 100 mg SR4/0.2 mL corn
oil/mice (4 mg/kg b.w.) by oral gavage on alternate days for
8 weeks. On the last day, the blood was collected within 2 h after
2.3. Cell lines and cultures
final dosage. To 50
standard in DMSO and 500
vortexed for 1 min and then spun down in a bench-top centrifuge
for 2 min. Then 400 L of the organic layer was collected,
evaporated to dryness and redisolved in 500 L methanol for
analysis. After an initial concentration test, 50 L were taken and
diluted to 2 mL with methanol for final analysis. A calibration curve
was built using a serial dilution of SR4 (from 0.1 pg/ L to 1 ng/ L)
in 125 pg/ L 6D-SR4.
SR4 and 6D-SR4 were separated by HPLC using an Agilent Zorbax
0.5 mm  150 mm C18, 5 m column with a flow rate of 50 L/min
m
L serum was added 0.5
mL 5 mg/mL 6D-SR4
Mouse (B16-F0) and human (A2058 and Hs600T) melanoma
cell lines were purchased from the American Type Culture
Collection (ATCC, Manassas, VA) in October 2011. Normal human
aortic vascular smooth muscle cells (HAVSMC) were kindly
authenticated and donated by Dr. Paul Boor, University of Texas
Medical Branch, Galveston, TX. All cells were cultured at 37 8C in a
humidified atmosphere of 5% CO2 in the appropriate medium:
RPMI-1640 (A2058) and DMEM (HAVSMC, B16-F0 and Hs600T)
medium supplemented with 10% heat-inactivated FBS and 1%
penicillin/streptomycin (P/S) solution. All the cells were also tested
for Mycoplasma once every 3 months.
m
L ethyl acetate. The samples were
m
m
m
m
m
m
m
m
and a gradient of 65% B to 95% B over 4 min, then 2 min at 95%B and
1 min back to 65%B. Buffer A was 5 mM ammonium formate and
buffer B was 5 mM ammonium formate in methanol. The elute from
the column was introduced into an Agilent 6410 triple quadrupole
tandem mass spectrometer by electrospray ionization (ESI) through
a capillary maintained at 4 kV, using 6 L/minnitrogennebulizing gas
at 350 8C and 15 psi, fragmentor voltage = 190 V. The following
transitions were monitored in positive ion mode: 344 > 127.1 and
2.4. siRNA mediated knock-down of GST
p
Cells were transfected with scrambled and GSTp siRNA at the
concentration of 10 g/mL in serum free medium, using Lipofecta-
m
mine 2000 (Invitrogen, Carlsbad, CA) for 3 h, according to the
manufacturer’s instructions. Excess siRNA was washed off with PBS

S.S. Singhal et al. / Biochemical Pharmacology 84 (2012) 1419–1427
1421
355 > 130.1 (collision energy = 28 V), 344 > 161.9 and 355 > 164.9
(C.E. = 20 V) for SR4 and 6dSR4. Quantitation was achieved using
Masshunter Quantitative Analysis 5 (Agilent Technologies) using a
linear calibration curve.
Immuno-histochemistry analyses were performed for Ki-67
expression (marker of cellular proliferation), CD31 (angiogenesis
marker), and pAMPK (cellular regulator of lipid and glucose
metabolism) from tumors in mice of control and SR4-treated
groups. Statistical significance of difference was determined by
two-tailed Student’s t test. p < 0.001, SR4-treated compared with
control. Immuno-reactivity is evident as a dark brown stain,
whereas non-reactive areas display only the background color.
Sections were counterstained with Hematoxylin (blue). Photo-
micrographs at 40Â magnification were acquired using Olympus
DP 72 microscope. Percent staining was determined by measuring
positive immuno-reactivity per unit area. Arrows represent the
area for positive staining for an antigen. The intensity of antigen
staining was quantified by digital image analysis using DP2-BSW
software. Bars represent mean Æ S.E. (n = 5); *p < 0.001 compared
with control.
2.9. In vivo xenograft studies
C57B mice (for syngeneic B16-F0 mouse melanoma model) and
Hsd: athymic nude nu/nu mice (for A2058 human melanoma mouse
xenografts model), were obtained from Harlan, Indianapolis, IN. All
animal experiments were carried out in accordance with a protocol
approved by the Institutional Animal Care and Use Committee
(IACUC). In each model, ten 10-weeks-old mice were divided into
two groups of 5 animals (treated with corn oil (vehicle), and SR4
compound 4 mg/kg b.w.). All animals were injected with 2 Â 106
melanoma cells suspensions in 100
mL of PBS, subcutaneously into
oneflankofeachmouse. Atthesametime, animalswererandomized
into treatment groups as indicated in the figures (Supplementary
Figs. 2 and 3). Treatment was started 10 days after the implantation
to see palpable tumor growth. Treatment consisted of 0.1 mg of SR4/
2.11. Statistical analyses
All data were evaluated with a two-tailed unpaired Student’s t
test and are expressed as the mean Æ SD. The statistical significance
of differences between control and treatment groups was determined
by ANOVA followed by multiple comparison tests. Changes in tumor
size and body weight during the course of the experiments were
visualized by scatter plot. Differences were considered statistically
significant when the p value was less than 0.05.
mice in 200
mL corn oil by oral gavage alternate day. Control groups
were treated with 200
mL corn oil by oral gavage alternate days.
Animals were examined daily for signs of tumor growth. Tumors
were measured in two dimensions using calipers. Photographs of
animals were taken at day 1, day 10, day 14, day 18, day 20, day 30,
and day 51 after subcutaneous injection, are shown for all groups.
Photographs of tumors were also taken at day 20 (for syngeneic
model), and at day 51 (for xenograft model).
3. Results
2.10. Histopathological examination of tumors for angiogenic,
proliferative and differentiation markers
3.1. Anti-proliferative and pro-apoptotic effect of SR4 in melanoma
The dichlorophenyl urea compound SR4 was synthesized by the
Drug Discovery Core Facility within City of Hope’s Comprehensive
Cancer Center [12]. The structure of SR4 is represented in Fig. 1
Control and SR4 treated B16-F0 and A2058 melanoma bearing
mice tumor sections were used for histopathologic analyses.
Fig. 1. Anti-proliferative and pro-apoptotic effects of SR4 in Melanoma. The chemical structure of 1,3-bis(3,5-dichlorophenyl)urea compound also called COH-SR4 (panel A).
Drug sensitivity assays were performed by MTT assay using SR4 at 96 h post-treatment to determine IC50. Values are presented as mean Æ standard deviation from two
separate determinations with eight replicates each (n = 16) (panel B). Colony-forming assay was performed and the colonies were counted using Innotech Alpha Imager HP as
detailed in Section 2. *p < 0.001 compared with control (n = 3, panel C). For TUNEL apoptosis assay, cells were grown on coverslips and treated with 10 mM SR4 for 24 h. TUNEL assay
was performed using Promega fluorescence detection kit and examined using Zeiss LSM 510 META laser scanning fluorescence microscope with filters 520 and 620 nm. Photographs
taken at identical exposure at 400Â magnification are presented. Apoptotic cells showed green fluorescence (panel D).

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S.S. Singhal et al. / Biochemical Pharmacology 84 (2012) 1419–1427
(panel A). The extent of melanoma cell survival was analyzed by
MTT assay following treatment with SR4 for 96 h. The SR4
treatment had a strong inhibitory effect on the survival of
melanoma cells [IC50: B16-F0 cell line-5 Æ 1 mM, Hs600 T cell
line-6 Æ 1 mM, and A2058 cell line-11 Æ 2 mM]. SR4 did not cause any
significant cytotoxicity in normal human aortic vascular smooth
muscle cells (HAVSMC) (Fig. 1B). The anti-proliferative effects of SR4
were further examined by colony forming assay. The SR4 (10 mM)
treatment resulted in 38 Æ 9%, 48 Æ 5% and 37 Æ 4% colony formation
in B16-F0, Hs600 T and A2058 melanoma cells. The SR4 treatment did
not significantly affect the colony forming ability of HAVSMC as the
HAVSMC cells displayed 96 Æ 4% colony forming potential compared
to respective untreated controls (Fig. 1C). The 10 mM of SR4 treatment
for 24 h induced apoptosis in B16-F0 and A2058 melanoma cells as
determined by enhanced DNA fragmentation in TUNEL apoptotic
assay (Fig. 1D). The cytotoxicity of SR4 in melanoma cells as evident
by MTT, clonogenic survival and apoptotic assays revealed that SR4 is
peroxidation like 4-hydroxy-2-nonenal (4-HNE) which eventually
leads to buffering of tumor-toxic oxidative stress and favors tumor
survival and proliferation in hypoxic environment [18]. Hence, we
investigated the effect of SR4 on the enzymatic activity of GSTs
towards 1-chloro 2,4-dinitro benzene (CDNB), a model substrate
routinely used for GST activity [19]. The SR4 treatment inhibited
the total GST activity to a significant extent in the B16-F0, Hs600T
and A2058 melanoma cells (Fig. 2A). The ability of SR4 to inhibit
GST activity in melanomas represents a potential mechanism that
contributes to decreased survival of melanoma cells following SR4
treatment (Fig. 2A).
3.3. GST
p depletion is associated with cytotoxicity in melanoma cells
SR4 treatment significantly decreased the GST activity in both
mouse and human melanomas cells (Fig. 2A). Given the significant
role of GST
impact of knock-down of GST
siRNA in B16-F0, Hs600T and A2058 melanoma cells. The knock-
down of GST was confirmed by Western-blot analyses (Fig. 2B).
The MTT assay revealed that GST -depletion itself decreased cell
p
in tumor progression (17), we further studied the
a
potential lead compound for melanoma. Also, independent
p
by transfection using GSTP1-1
cytotoxicity testing from the NCI-60 DTP Human Tumor Cell Line
Screen further confirmed the potential anti-cancer activities of SR4
against several melanoma cell lines (Supplementary Fig. 1).
p
p
growth by ꢀ35–46% (Fig. 2C, inset), and sensitized to SR4
3.2. Effect of SR4 on GST activity in melanoma cells
significantly by decreasing the IC50 to almost half (Fig. 2C). Taken
together, these studies suggest that SR4 targets GST
p activity and
GSTs are a class of phase II detoxifying enzymes, which mediate
drug resistance by detoxifying administered chemotherapy drugs
for efflux out of cells by transport proteins. The over-expression of
GSTs is associated with malignant progression of many cancers
including melanoma, lung and prostate cancers [10,16,17]. GSTs
mediate glutathione conjugation of toxic end products of lipid
that GST inhibition further sensitizes to the growth inhibitory
p
effects of SR4 in melanoma. The anti-proliferative effect of SR4
was further examined by cell cycle FACS analysis. SR4 treatment
caused G2/M phase arrest in both B16-F0 mouse and A2058
human melanoma cells (ꢀ50% cells accumulated in G2 phase)
(Fig. 2D).
Fig. 2. Effect of SR4 on cell cycle progression in melanoma. GST activity towards 1-chloro 2,4-dinitro benzene (CDNB) and its inhibition by SR4 was performed in 28,000 g
crude supernatant prepared from B16-F0, Hs600T and A2058 cells. Human liver purified GST was used as a control (inset). The inhibitory effect of SR4 on GST was studied at a
fixed concentration of GSH and CDNB (1 mM each) and varying concentrations of inhibitor. The enzymes were pre-incubated with the inhibitor for 5 min at 37 8C prior to the
addition of the substrates (panel A). Depletion of GST
melanoma cell lines using Lipofectamine 2000 (Invitrogen). After knock-down of GST
stripped and reprobed for -actin as a loading control. Results were quantified by scanning densitometry: C, control siRNA; T, GST
p
by siRNA and its effects on cell survival by MTT assay: GST
by siRNA, the level of GST was detected by Western blot analyses. Membranes were
siRNA (panel B). MTT assay in GST siRNA
p siRNA or a scrambled control was transfected into
p
p
b
p
p
transfected cells were performed 96 h after SR4 treatment. The values are presented as mean Æ SD from two separate determinations with eight replicates each (n = 16) (panel
C). GSTp-depletion itself caused decreased in cell growth (panel C, inset), * p < 0.01 compared to control. Inhibitory effect of SR4 on cell cycle distribution was determined by
fluorescence activated cell sorting (FACS) analysis (panel D). The experiment was repeated three times and similar results were obtained.

S.S. Singhal et al. / Biochemical Pharmacology 84 (2012) 1419–1427
1423
3.4. Analyses of SR4 in mice serum
Next, we assessed the absorption of orally administered SR4 in
mice. The C57 B mice were treated with 0.1 mg/mice (4 mg/kg
b.w.) of SR4 on alternate day for 8 weeks. The blood was collected
within 2 h of the dosing on the final day of treatment and further
processed for MS analyses as described in Section 2. LC–MS/MS
analysis of SR4 treated mice serum revealed that SR4 is effectively
absorbed after oral dosage and it reaches a serum concentration of
342 Æ 44 mg/L (equivalent to 1 Æ 0.22 mM) (Fig. 3A–D).
3.5. Analyses of blood chemistries in control and SR4 treated mice
To evaluate the potential toxicity of SR4 on animals, C57 B mice
were treated with 0.1 mg/mice (4 mg/kg b.w.) of SR4 or with
vehicle on alternate day for 2 weeks (n = 6). After the treatment
period, blood and plasma/serum were isolated and analyzed. Oral
administration of SR4 showed no significant differences on key
blood and metabolic profiles as compared with vehicle-treated
mice. The plasma alanine transaminase (ALT) and alkaline
phosphatase (ALP) were moderately higher in SR4-treated mice
(p < 0.05), while the levels of two other liver enzymes; aspartate
transaminase (AST) and lactate dehydrogenase (LDH) were similar
with control mice (Supplementary Table 1).
3.6. Anti-neoplastic effect of SR4 in vivo on melanoma progression
C57B mice (for syngeneic B16-F0 mouse melanoma model) and
Hsd: athymic nude nu/nu mice (for A2058 human melanoma
mouse xenografts model), were used for testing the impact of oral
administration of SR4 on melanoma progression in vivo models. In
each model, ten 10-weeks-old mice were divided into two groups
of 5 animals (treated with corn oil (vehicle), and SR4 compound
(4 mg/kg b.w.). All animals were injected with 2 Â 106 melanoma
cells suspensions in 100
mL of PBS, subcutaneously into one flank
of each mouse. Treatment was started 10 days after the
implantation of melanoma cells. Treatment consisted of 0.1 mg
of SR4/mice in 200
Control groups were treated with 200
m
L corn oil by oral gavage on alternate days.
L corn oil by oral gavage
m
alternate day. In our studies, the SR4 treatment was tolerated well
by the mice without any weight loss compared with age-matched
controls (Fig. 4A). Animals were examined daily for signs of tumor
growth. Tumors were measured in two dimensions using calipers.
The SR4 treatment lead to significant reduction in the tumor
burdens in the treated groups [B16-F0 syngeneic melanoma
model: 2.36 Æ 0.2 g vs. 1.07 Æ 0.2 g in control and SR4 treated groups,
respectively on day 20. A2058 human melanoma xenograft model:
1.91 Æ 0.3 g vs. 0.7 Æ 0.1 g in control and treated and groups,
respectively, on day 51] (Fig. 4B). Photographs of animals were
taken at day 1, day 10, day 14, day 18, day 20, day 30 and day 51 after
subcutaneous injection, are shown for all groups. Photographs of
tumors were also taken at day 20 (for syngeneic model), and at day 51
(for xenograft model) (Supplementary Figs. 2 and 3). The time course
analyses of SR4 treatment revealed a substantial inhibition of tumor
progression in both syngeneic and xenografts models of melanoma
whereas uncontrolled growth was observed in untreated controls
(Fig. 4C). In parallel xenografts studies, we also used 20 mg/kg b.w.
SR4 to see the better regression and any toxicity. Higher dosage of SR4
caused no further improvement in tumor regression and no toxicity
was observed (Supplementary Fig. 3). The SR4 treated animals with
B16 melanoma survived for 50 Æ 5 days, while all animals treated
with vehicle only were censored by day 20 Æ 2. The SR4 treated
animals with A2058 melanoma were still alive at 88 days, while all
animals treated with vehicle only were censored by day 51 Æ 3. These
results indicated that SR4 administration inhibits melanoma growth
and prolongs survival without causing side effects.
Fig. 3. Measurement of serum levels of SR4. The serum levels of SR4 in control and
SR4 treated mice was examined by LC–MS/MS analyses. The C57 B mice were
treated with 0.1 mg/mice (4 mg/kg b.w.) of SR4 on alternate day by oral gavage for
8 weeks. On the final day of treatment blood was collected within 2 h of final
dosage. The samples were processed and analyzed for serum levels of SR4 as
described in Section 2. The panels represent multiple reaction monitoring (MRM)
mode chromatograms of 6D-SR4 standard 5 pg/mL (panel A), control serum (panel
B) and SR4 treated mice serum (panel C). The bar diagram represents the
quantification (mean Æ SD) of SR4 in mice serum (n = 3); ND, not detectable (panel D).
3.7. Impact of SR4 on the markers of proliferation and angiogenesis
Following the in vivo animal studies, the histopathological
examination of paraffin-embedded tumor xenograft sections by
H&E staining revealed that SR4 reduces the number of tumor blood
vessels and restores the normal morphology when compared to
controls (Fig. 5). SR4 treatment decreased the levels of proliferation
marker, Ki 67 and angiogenesis marker, CD31 as revealed by ABC
staining. AMPK is a critical cellular protein which senses the low
energy status of cells and inhibits proliferation and survival of cells
[20]. SR4 treatments lead to increase in the levels of pAMPK in
tumor sections which provides corroborative evidence for the
induction of anti-tumor effects in in vivo models of melanoma.

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S.S. Singhal et al. / Biochemical Pharmacology 84 (2012) 1419–1427
Fig. 4. Effect of oral administration of SR4 on melanoma progression in mice. The C57B mice and Hsd: Athymic nude nu/nu mice were obtained from Harlan, Indianapolis, IN.
In each model, ten 10-weeks-old mice were divided into two groups of 5 animals (treated with corn oil (vehicle), and SR4 compound 4 mg/kg b.w.). All animals were injected
with 2 Â 106 melanoma cells suspensions in 100
m
L of PBS, subcutaneously into one flank of each mouse. Treatment was started 10 days after the implantation to see palpable
L corn oil by oral gavage alternate day. Control groups were treated with 200 L corn oil by oral gavage
tumor growth. Treatment consisted of 0.1 mg of SR4/mice in 200
m
m
alternate day. Animals were examined daily for signs of tumor growth and body weights were recorded (panel A). Photographs of animals were taken at day 1, day 10, day 14,
day 18, day 20, day 30, day 40, and day 60 after subcutaneous injection, are shown for all groups (Supplementary Figs. 2 and 3). Weights and photographs of tumors were also
taken at day 20 (for syngeneic model), and at day 51 (for xenograft model). Photographs of tumors were also taken at day 20 (for syngeneic model), and at day 51 (for
xenograft model) (panel B). Tumors were measured in two dimensions using calipers and time-course analysis of tumor regression was performed during the study (panel C).
Fig. 5. Histopathologic analyses of tumor sections after SR4 treatment. Control and SR4 treated B16-F0 and A2058 melanoma bearing mice tumor sections were used for
histopathologic analyses. Immuno-histochemistry analyses for Ki-67, CD31, and pAMPK expression from tumors in mice of control and SR4-treated groups. Statistical
significance of difference was determined by two-tailed Student’s t test. p < 0.001, SR4-treated compared with control. Immuno-reactivity is evident as a dark brown stain,
whereas non-reactive areas display only the background color. Sections were counterstained with Hematoxylin (blue). Percent staining was determined by measuring
positive immuno-reactivity per unit area. Arrows represent the area for positive staining for an antigen (panel A – histopathology of resected B16-F0 syngeneic mouse
melanoma tumors; panel B – histopathology of resected A2058 human melanoma tumors). The intensity of antigen staining was quantified by digital image analysis. Bars
represent mean Æ S.E. (n = 5); * p < 0.001 compared with control.

S.S. Singhal et al. / Biochemical Pharmacology 84 (2012) 1419–1427
1425
Fig. 6. Effect of SR4 on signaling proteins in vivo models of melanoma. Western-blot analyses of signaling proteins in tumor tissue lysates in control and SR4 treated
experimental groups (panel A – Western blot of lysates from resected B16-F0 syngeneic mouse melanoma tumors from C57 B mice; panel B – Western blot of resected A2058
human melanoma tumors from nu/nu nude mice xenograft model). The bar diagrams represent the fold change in the levels of proteins as compared to controls as determined
by densitometry. Dotted line represents no significant change as observed with control.
3.8. Effect of SR4 on the expression of tumor proteins
melanoma is one of the top ten cancers with an increasing risk
on incidence in Caucasian populations. The current therapeutic
options for melanomas are limited. The drugs used in melanoma
therapy include temozolomide, dacarbazine, hydroxyureas and
interleukin-2 along with the recent approval of vemurafenib for
The SR4 treated groups had high levels of the cleaved PARP
compared to untreated controls, which is in accordance with the
observed apoptotic effects of SR4 in vitro melanoma cultures. Akt is
a critical signaling protein that transduces the proliferative signals
from upstream integrins and growth factor receptors [21]. The SR4
treatments lead to an increase in the levels of PARP cleavage along
with decrease in the levels of Akt and pAkt (S473). The cellular
levels of vimentin and fibronectin determine the extent of
migration and proliferation in melanoma cells [22,23]. SR4
treatments lead to decreases in the expression of vimentin and
fibronectin which are associated with invasive progression of
melanomas. SR4 treated groups had an enhanced expression of
pro-apoptotic protein Bim along with a parallel decrease in the
levels of anti-apoptotic protein Bcl2. The expression of cell cycle
regulatory proteins CDK4 and Cyclin B1 was decreased following
SR4 treatment. Our results regarding G2/M phase arrest was
observed consequent to SR4 treatment are in accordance with
some of the published studies which indicate that that anti-cancer
compounds such as apigenin and thimerosal which also cause
inhibition of CDK4 along with cyclin B1 [24,25]. In accordance with
the histopathological examination, the levels of pAMPK (T172) were
enhanced in SR4 treated groups compared to controls in both B16-
F0 and A2058 melanoma (Fig. 6).
advanced and metastatic melanomas over-expressing BRAF V600
E
mutation [26]. However, the response to chemotherapy is limited
and resistance is prominent in many cases of melanomas as
reported by several studies [27,28]. Thus, new molecules capable
of being effective in inducing anti-tumor effects along with
regulatory effects on tumor signaling proteins of specific signifi-
cance in melanoma progression and therapeutic resistance present
as significant candidates for further stages of clinical drug
development. In this regard, the results from our investigations
on the anti-cancer effects and mechanisms of action of SR4 assume
significance.
The efficacy of radiotherapy relies on the oxidative stress
induced generation of hydroxyl radicals and consequent DNA
damage. The oxidative stress induced due to radiotherapy also
enhances the generation of cellular levels of lipid peroxidation
products which contribute to the toxic effects of radiotherapy [15].
The products of lipid peroxidation like 4-hydroxy-nonenal (4-HNE)
and chemotherapy drugs, are rapidly conjugated to glutathione
(GSH) by intracellular GSTs to form GSH adducts (electrophilic-
conjugates or GS-E). The formation of GS-E is a rate limiting step in
the process of detoxification of toxic products of lipid peroxidation
[29]. As, the expression of GSTs is increased in malignant
melanoma, the inhibition of GST activity by SR4 is a significant
finding for further studies on SR4 towards possibly potentiating
the effects of radiotherapy and chemotherapy. SR4 treatment
effectively induced the cell cycle arrest in melanoma cells. The oral
4. Discussion
The development of novel interventional strategies and
effective chemotherapies for melanomas remains one of the
frontier challenges in clinical oncology, given the fact that

1426
S.S. Singhal et al. / Biochemical Pharmacology 84 (2012) 1419–1427
administration of SR4 was well tolerated without any overt
toxicity along with inducing effective tumor inhibition in both
syngeneic and xenografts mouse models of melanoma.
three fold increase in the invasion through basement membrane
matrix as well as migration in vitro [23]. The over-expression of
vimentin is also associated with hematogenous metastases of
melanoma cells [44]. Thus, the inhibition of fibronectin and
vimentin by SR4 assumes translational significance.
Our studies indicate that SR4 is tolerable and effective anti-
cancer agent with no overt toxicity in vivo models of melanoma.
One of the members of this class of compounds namely 1,3-bis
(3,4-dichlorophenyl) urea, a close relative of SR4, was studied
earlier by Proctor et al. as compound II, was demonstrated to be an
electron transport inhibitor that reduced Staphylococcus aureus
hemolytic activity and to protected cultured endothelial cells from
lysis [30]. In their studies on compound II, they found that the
effective dose for the inhibition of bacterial respiration was 50
times lower than the concentration required to cause similar
inhibition of rat mitochondrial respiration and also human and
animal exposure studies showed a lack of acute toxic responses to
compound II.
The analyses of the expression of tumor proteins consequent to
SR4 treatment revealed the impact on significant nodes of
melanoma proliferation, invasion and metastases. In melanomas,
the tumor growth phases include ‘‘radial growth phase (RGP)’’ and
‘‘vertical growth phase (VGP)’’. Melanomas behave as highly
metastatic tumors after invading the dermis and hence, malignant
melanomas in RGP are usually surgically excised but post-surgical
recurrence is common from satellite lesions, which become
aggressive once the primary tumor is removed [31]. Thus, the
tumor proteins which regulate the transition from RGP to VGP in
melanoma hold significant promise in developing targeted
therapies. In a study, reverse phase protein array analyses of
resected melanoma tissues revealed that the expression levels of
Akt is high in advanced and metastatic melanomas [32]. The over-
expression of Akt leads to conversion of RGP to VGP in melanoma
[33]. Akt activation is known to mediate resistance to microtubule
targeting agents as well as BRAF and MEK inhibitor AZD6244 or
Vemurafenib [34,35]. Thus, the inhibition of Akt phosphorylation
by SR4 represents an interesting finding for targeting the
progression and drug-resistance in melanomas.
The Bcl2 family proteins play a vital role in the survival and
apoptosis of various cancers including melanoma [36]. The anti-
apoptotic protein Bcl-2 is a natural and preferred target that needs
to be inhibited for successful melanoma interventions. Among the
human epidermal cells, melanocytes are the only cells that
constitutively express Bcl2 and the Bcl2 expression is enhanced
with malignant transformation [37,38]. Also, Bcl-2 knockout mice
display rapid greying of hair because of decreased survival of
follicular melanocytes [39]. Elevated levels of Bcl2 are associated
with enhanced survival and chemotherapy resistance in melano-
ma. The expression of pro-apoptotic protein Bim, a protein known
to antagonize the functions of Bcl2, is decreased in cutaneous
melanomas and this is associated with poor 5-year survival [40].
Thus, the decreased expression of anti-apoptotic Bcl-2 and
increased expression of pro-apoptotic Bim reinforce the relevance
of mechanisms of action of SR4 in causing apoptosis in melanoma.
The inhibition of the expression of cell cycle regulatory proteins
cyclin B1 and CDK4 provides corroborative rationale for the impact
of SR4 on cell cycle progression in melanoma. The inhibition of
cyclin B1 is also associated with response to gamma-irradiation
therapy which, along with the inhibition of GST activity by SR4,
provides rationale for further combinatorial studies with radio-
therapy [41]. The epithelial mesenchymal transformation (EMT)
during the transformation and metastasis leads to expression of
transcription factors like snail and slug, which in turn enhance the
expression of fibronectin and vimentin [42]. The proliferating and
invading melanoma cells express a variant of fibronectin which can
be rapidly secreted out of cells and can activate haptotactic
migration of melanoma cells [43]. The over-expression of vimentin
in a weakly invasive A 375P melanoma cell line lead to a two-to
The AMPK signaling regulates vital processes of proliferation
and survival in many cancers. AMPK is activated by high levels of
AMP/ATP ratio or decreased cellular energy status which leads to
decreased metabolic activity and proliferation consequent to
AMPK activation in energy deficient cells [45]. Cancer cells
suppress the activation of AMPK in spite of imbalance between
cellular energy vs. rate of proliferation and thus sustain active
growth and invasion [20]. The BRAF V600E mutations lead to
constitutive inhibition of AMPK (T172) phosphorylating kinase
LKB1, which results in low levels of AMPK activation in melanoma
cells [46]. Thus, the enhanced phosphorylation of AMPK (T172) by
SR4 as revealed by both histopathological analyses of resected
tumor sections (Fig. 5) and Western blot analyses of tumor tissue
lysates following SR4 treatment is another significant mechanistic
finding in the context of melanoma pathogenesis and progression
(Fig. 6).
In summary, the increasing incidence of melanomas along with
high rates of therapeutic refractoriness to conventional modes of
clinical interventions necessitate characterization and validation
of novel anti-cancer compounds for mono and combinatorial
therapy in melanomas. In this regard, the inhibitory effects and
mechanisms of action of SR4 as validated by both syngeneic and
transgenic mouse models of melanoma provide strong and
relevant mechanistic rationale for further translational studies.
Author’s contributions
Sharad S. Singhal contributed to research data and wrote
manuscript; James Figarola contributed to research data. Jyotsana
Singhal contributed to research data; Kathryn Leake contributed to
research data; Lokesh Nagaprashantha contributed to research
data; Christopher Lincoln contributed to research data; Gabriel
Gugiu contributed to research data; David Horne contributed to
review/edit manuscript; Richard Jove contributed to review/edit
manuscript; Sanjay Awasthi contributed to discussion and review/
edit manuscript; Samuel Rahbar contributed to discussion and
review/edit manuscript.
Acknowledgements
This work was supported in part by USPHS grant CA 77495 (to
SA) and CA 115674 (to RJ).
Appendix A. Supplementary data
Supplementary data associated with this article can be found, in
the online version, at http://dx.doi.org/10.1016/j.bcp.2012.08.020.
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